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The cheese sandwich's journey begins when it is taken into the mouth, this process is ingestion. Whilst in the mouth, the large particles are broken down into much smaller particles, by chewing, which also mixes the food with saliva which contains a carbohydrase enzyme. This amylase digests the starch in the sandwich to glucose. The saliva is produced in the three salivary glands. These are called, parotid, sublingual and submandibular.
When the sandwich is swallowed, the food reaches the oesophagus. The reason why the food goes down the oesophagus rather than the windpipe is that a flap called the epiglottis drops over the windpipe stopping the sandwich particles from getting into the lungs.
The oesophagus links the mouth and the stomach together, so the particles of the cheese sandwich enter the stomach organ next, where the food mixes with a protease enzyme which digests protein. The carbohydrates which were digested in the mouth were used in the human body for energy. There is high of starch content in the bread of the cheese sandwich. The carbohydrates are like a body fuel. Professional sportsmen and women need a diet high in carbohydrates because they are active and need energy. Protein, on the other hand which will be found in cheese is used for building cells. Proteins are vital for growth and to repair damaged areas of the human body. Hydrochloric acid is present in the stomach to kill bacteria. It has a low pH, and the enzymes produced by the stomach work best in acidic conditions.
The food empties from the stomach into the duodenum where it is mixed with enzymes and salts produced by the liver and enzymes made in the pancreas. The pancreas secretes three main enzyme types: protease enzymes, carbohydrase enzymes and also lipase enzymes run into the duodenum at the top the small intestine, where it continues its journey. The fats in the sandwich, which are in the butter and the cheese, are also used for energy. Fats act as a store of energy, which are used only when the carbohydrates run out. If there is too much fat it will store up and become obvious to other people that too much is stored. Bile is also alkaline to give the correct pH for the enzymes in the small intestine. The small intestine produces more enzymes to further digest proteins, fats and carbohydrates.
Around the oesophagus and throughout the small intestine (duodenum and the 20ft long ileum) are circular muscles which press the sandwich particles along in a contacting and relaxing motion, so the food gets pressed along. This contracting and relaxing motion is called peristalsis. The muscles responsible for peristalsis need to be regularly exercised. The way these muscles get exercised is not just by normal foods going down and the muscles pressing the particles along. They become stronger by fibre going down and because fibre is difficult to digest the muscles have to press hard and like any other muscles in the human body, they become stronger when they are worked on.
In the small intestine are millions of villi. These are small, but visible to the human eye, finger like structures which line the small intestine and are perfect for absorbing the food. The main reason why villi are so good at absorbing food is that they have a thin layer of outer cells, which act as a cell, called micro-villi. Whilst villi are structures, micro-villi are tiny cells outside the villi. The villi also have a high blood supply and provide a big surface area for absorption. In the small intestine the food is absorbed through the gut wall into the blood, which takes it around the body to wherever the particles are needed. The job of the enzymes in the small intestine is also to break down the foods.
Chemical digestion, under the control of enzymes throughout the digestive system, takes up to twenty four hours to complete, depending on what the is being digested. Ultimately this is very thorough. Mechanical digestion on the other hand is done mostly in the mouth (where the teeth are biting down the cheese sandwich into much smaller pieces), but also in the stomach which churns food through muscular action. Mechanical digestion happens quickly but is less thorough at breaking down food. Only particles which have been chemically digested are able to pass through into the blood through the cell wall.
The next part of the digestive system is the large intestine. Here the water is absorbed so it is not wasted and passed out with the faeces. Water plays a major part in digestion as 75% of our body is made up of water. The rectum stores the waste products and non-digestible food and completes the re-absorption of water. Finally, when convenient, the waste faeces are passed out of the body through the anus. This is called egestion.